[23.01] Evidence for Gradual External Reconnection Leading to Explosive Eruption of a Solar Filament

A. C. Sterling (NASA/MSFC/NSSTC/UAT), R. L. Moore (NASA/MSFC/NSSTC)

We observe a slowly-evolving quiet region solar eruption of
1999 April~18, using images in 195~Å\ Fe~{\sc xii} from
EIT on SOHO, and in soft X-rays from SXT on
Yohkoh. We examine dimmings and brightenings in difference
images, where an early image is subtracted from later
images, for evidence of the eruption mechanism. A filament
rose slowly at about 1~km~s-1 for six hours before
being rapidly ejected at about 10~km~s-1, leaving flare
brightenings and post-flare loops in its wake. SOHO
MDI data show that the eruption occurred in a quadrupolar
region, with the filament location splitting the four
magnetic sources. During the slow rise, subtle EIT dimmings
occur between the filament and one of the remote magnetic
regions. Concurrently, soft X-ray brightenings occur between
the filament and either remote magnetic region. Both of
these effects suggest temperature enhancements in magnetic
loop systems on either side of the filament prior to
eruption. Pre-eruption SXT dimmings occur on either side of
and very close to the slowly rising filament, indicating
expansion of enveloping magnetic loops. At the start of the
rapid ejection, intense dimmings occur at the locations
evacuated by the filament, and brightenings occur underneath
the fast-moving but still low-altitude filament. We consider
two models for explaining the eruption: ``breakout,'' which
says that reconnection occurs high above the filament prior
to eruption, and ``tether cutting,'' which says that the
eruption is driven by reconnecting field lines beneath the
filament. We find that pre-eruption evolution is consistent
with breakout. Tether cutting-type reconnection occurs
during the rapid ejection, but our data are not complete
enough to determine whether that reconnection is the primary
cause of the fast-phase onset.